The present invention relates in general to devices incorporating a microphone and a sound or noise source in a single enclosure, such as handsfree or speaker phone systems, telephones, and more particularly to a circuit for improved separation between microphone and speaker.
It is a requirement of a handsfree or speaker phone design to provide sufficient acoustic separation between the microphone and the speaker. For systems where the microphone and speaker are to be in the same enclosure this requirement can present a challenge. This is a problem for both half-duplex and full-duplex systems, where the acoustic coupling of the speaker output to the microphone input must be minimized to maintain stability in the system.
One component of this coupling is mechanical (i.e. through the structure of the enclosure). The other component is transmitted via air. This coupling is usually greater at or about the resonance frequency of the speaker.
Much acoustic, industrial and mechanical design effort can be expended in design of the enclosure in order to minimize the separation, but this does not usually occur because of cost and the limited time within which to supply a product to market. Sometimes the acoustic aspect of an enclosure are dealt with after other aspects of the enclosure are finalized, thus reducing the opportunity to minimize coupling.
The use of unidirectional microphones can reduce coupling, but it is difficult to design unidirectional microphones into a telephone type of enclosure because they require the sound to be present at the front and back of the microphone. Also, a unidirectional microphone restricts the user to one position in front of the microphone.
It is an object of an aspect of the present invention to provide a simple and inexpensive means for providing improved separation between the speaker and microphone in a handsfree or speaker phone system where both transducers are located in the same enclosure.
According to the present invention, improved acoustic separation is provided in an handsfree or speaker phone enclosure by means of providing a second inexpensive omnidirectional microphone (referred to herein as the speaker microphone) which is placed inside the enclosure to primarily pick up sound from the speaker, and a circuit for subtracting this sound from the sound picked up by the voice microphone which is positioned to pick up sound occurring outside of the enclosure.